1. Field of the Invention
This invention relates to an apparatus for detecting a failure of a working tool of a machine tool and more particularly to a working tool failure detection apparatus capable of accurately detecting a failure of even a small size working tool. Although the following detailed description of the invention will be given by way of a drilling machine for printed circuit boards, it should be understood that applicability of the present invention is in no way limited thereto but is valid for other types of machining apparatus.
2. Description of the Related Arts
In a machine tool such as a drilling machine for circuit boards, a board to be worked is placed on a horizontally movable table, and a drill is actuated to bore a desired hole in the board while the table is moved under the direction of a controller programmed with predetermined working sequence and working positions. The workable circuit board consists of a base layer made of bakelite or glass epoxy resin and a copper foil conductor fixedly formed on the base layer. The recent demand for high density packaging of electronic circuits has required an increasing reduction in the size of holes to be bored in the board. Accordingly, the working tool or drill trends toward an increasing reduction in its diameter and as a result, a chance of failure of the drill is on the increase. In spite of the above circumstances, the smaller the drill diameter, the more it becomes difficult to rapidly detect a failure of the drill.
A conventional apparatus for detection of a failure especially breakage of a drill is disclosed in, for example, Japanese Utility Model Unexamined Publication No. 56-33151. According to this proposal, an amount of chips produced by drilling is optically detected so as to determine an occurrence of a break of the drill when the chip amount decreases. With this apparatus, however, the break detection is possible only when chips are produced in a great amount but is impossible in the case of a small chip production amount or no production of chips. Also disclosed in U.S. Pat. No. 3,912,925 is a drill breakage detection apparatus wherein light is irradiated on a drill so that transmitted light can be detected by a photodetector when the drill is broken. This apparatus is highly effective when changes in the quantity of light is dependent on the presence or absence of a drill portion under light irradiation are appreciably great but faces difficulties in discriminating breakage of the tool when the drill portion is small because quantities of light intercepted thereby are small, resulting in small changes in the quantity of light. Moreover, the drill and the photodetector are coaxially aligned with a light source in a narrow, elongated space to increase the scale of the apparatus, leading to incompatibility with the demand for the size reduction of the tool itself.